The present invention relates to a cell voltage measuring device for a cell module comprising a plurality of cells connected together in series.
Attention has been directed in recent years to environmental technology with consideration given to global environmental problems such as ozone hole and global warming. In the motor vehicle industry, efforts have been devoted to the development of electric motor vehicles with diminished carbon dioxide emissions. As power sources for such vehicles, cell modules having a great capacity are used in which secondary cells, typical of which are lithium ion cells or nickel hydrogen cells, are connected in series.
Secondary cells, typical of which are lithium ion secondary cells, cause undesirable troubles such as liquid leakage or heat generation if overcharged or overdischarged, so that each cell needs to be checked for voltage by monitoring. Furthermore, handling a cell module of large capacity having great energy requires safety measures such as electrical insulation of the module and the circuit in the vicinity thereof. Accordingly, techniques for measuring the voltage of each cell accurately with safety are important for cell modules of large capacity for use as power sources for electric motor vehicles.
Cell voltage measurement of such cell modules having a great capacity is conventionally done, for example, by dividing the module into blocks of several cells, multiplexing the cell voltages of each block and converting the voltages to digital values, and obtaining cell voltages in terms of digital values as shown in FIG. 2. However, this method requires many AD converters, and the power sources for the AD converters need to be insulated, consequently entailing the problem of necessitating a complex circuit and a higher cost.
Accordingly, the cell voltage measuring method with use of so-called flying capacitors has attracted attention in recent years. With reference to FIG. 3 showing an example of arrangement of flying capacitors, indicated at 31 is a cell module, at 32 potential holding means, at 33 cell voltage measuring means, and at 34 control means. With the method of FIG. 3, the switches of the potential holding means 32 are turned on first to cause the capacitors of the means 32 to hold potentials of the cell module 31, and the switches are thereafter turned off to electrically insulate the cell module 31 from the potential holding means 32. The capacitor corresponding to the cell to be checked for voltage is connected to an A/D converter to measure the cell voltage by the measuring means 33. The cell voltage of the cell module 31 can be measured by this method, with the module 31 electrically insulated from the cell voltage measuring means 33.
FIG. 4 shows another example of arrangement of flying capacitors. Indicated at 41 is a cell module, at 42 potential holding means, at 43 cell voltage measuring means, and at 44 control means. With the illustrated construction like that of FIG. 3, the cell voltage can be measured based on the potential held in the capacitor of the potential holding means 42, with the cell module 41 electrically insulated from the cell voltage measuring means 43.
However, the construction shown in FIG. 3 requires mounting of many switches, necessitating complex wiring and a higher cost. Although the construction shown in FIG. 4 is smaller in the number of switches needed, all the capacitors constituting the potential holding means 42 are connected to one another in series, consequently entailing the need for the cell voltage measuring means 43 to handle a high voltage, so that the device is not useful practically. For example, in the case where 40 lithium ion cells are connected in series, a voltage of 3.6 Vxc3x9740=144 V must be handled, whereas it is difficult to measure with the same accuracy the cell voltages increasing over a wide range of from 0 to 144 V with an increment of about 3.6 V.
Accordingly, an object of the present invention is to provide a cell voltage measuring device which is reduced in the number of switches, lower in the voltage to be handled and adapted to measure the voltages of a cell module as electrically insulated.
The present invention provides a cell voltage measuring device for a cell module 11 wherein the component cells are theoretically divided into a plurality of (n) cell blocks 111 to 11n. A plurality of potential detecting lines extending from respective potential detecting points of each of the cell blocks are provided with potential holding means 12 for holding the potentials of the potential detecting points of the cell block, and cell voltage measuring means 14 for measuring the voltage of the cells based on the potentials of the potential detecting points held by the potential holding means 12. The potential holding means 12 and the cell voltage measuring means 14 have their operations controlled by control means 17.
The potential holding means 12 comprises: a plurality of (n) pre-switch blocks 121 to 12n connected to the plurality of (n) cell blocks 111 to 11n respectively and each comprising a plurality of switches capable of opening or closing the potential detecting lines extending from the cell block, and
a plurality of (n) capacitor blocks 131 to 13n connected to the plurality of (n) cell blocks 111 to 11n by way of the plurality of (n) pre-switch blocks 121 to 12n respectively and each comprising a plurality of capacitors for holding the potentials of the potential detecting points of the cell block. The voltage measuring means 14 comprises:
a plurality of (n) post-switch blocks 141 to 14n connected to the plurality of (n) cell blocks 111 to 11n respectively by way of the potential holding means 12 and each comprising a plurality of switches for opening or closing the potential detecting lines extending from the cell block, and
a voltage measuring circuit for selecting voltage signals for each cell block from among those obtained by the potential detecting lines via the post-switch blocks 141 to 14n to measure the voltage of each cell constituting the cell block.
The voltage measuring circuit can be composed of a difference computing circuit 150 for calculating the potential difference between two potential detecting lines extending from electrode terminals of each cell and included among the potential detecting lines from each cell block, and an AD converter 160 having connected thereto output terminals of the computing circuit 150.
With the cell voltage measuring device of the invention for the cell module, the cell module is divided into a plurality of cell blocks, and flying capacitors are provided for each cell block. This feature reduces the number of switches, lowers the voltage to be handled and permits voltage measurement with the module in an electrically insulated state, at the same time.